The present invention is discussed in the following largely with reference to the medical industry, but the present invention is applicable to a variety of contexts and environments, each of which may utilize or benefit from an improved tomographic imaging system, for example, archaeology, biology, geophysics, materials science, electron microscopy, security scanning, industrial nondestructive testing, astronomy and others.
Tomography is imaging by sections or sectioning to convey internal structures of a solid object, for example the human body or the earth. Slices of the object are viewed without physically cutting the object. A device used in tomography is called a tomograph. A tomograph generates a tomogram, or image.
The image, or tomogram, can be achieved by tomography applications such as acoustic tomography, atom probe tomography (APT), computed tomography (CT), confocal laser scanning microscopy (LSCM), cryo-electron tomography (Cryo-ET), electrical capacitance tomography (ECT), electrical resistance tomography (ERT), electrical impedance tomography (EIT), functional magnetic resonance imaging (fMRI), magnetic induction tomography (MIT), magnetic resonance imaging (MRI), formerly known as magnetic resonance tomography (MRT), neutron tomography, optical coherence tomography (OCT), optical projection tomography (OPT), process tomography (PT), positron emission tomography (PET), quantum tomography, single photon emission computed tomography (SPECT), seismic tomography, and X-ray tomography.
Electromagnetic and acoustic tomography requires the inversion of a wave equation. More modern variations of tomography involve gathering projection data from multiple directions and feeding the data into a tomographic reconstruction algorithm processed by a computer in order to create a tomographic image. The reconstruction algorithm includes an inversion technique.
Various inversion techniques for wave equations arising from electromagnetic and acoustic scattering imaging systems have been developed since the early 1980s. Scattering is a general physical process whereby some forms of radiation, such as light, sound or moving particles, for example, are forced to deviate from a straight trajectory by one or more non-uniformities in the medium through which it passes. It is the inverse problem to the direct scattering problem that determines the characteristics of an object such as its shape and internal constitution from measurement data of radiation or particles scattered from the object.
A substantial amount of research during the last decade or so has focused on a full non-linear inversion problem, or a non-linear inversion technique. These inversion techniques have matured to the point for possible use in biomedical imaging and identification, and experimental systems have already been created that show good potential for electromagnetic imaging of limbs and breast tumors.
An advantage of the full non-linear inversion technique, as opposed to a linearized technique, is that a quantitative inversion of material parameters such as conductivity and permittivity significantly improves solving the clinical identification problem, e.g., tumor or no tumor, and makes the non-linear inversion technique much more useful for biomedical applications.
Large classes of inversion techniques for wave-type equations are formulated as non-linear optimization problems which are then solved using an iterative method. Most of these methods require the use of a Green's function and, typically, these methods have been implemented using the Green's function associated with a scatterer located in an unbounded homogeneous region. However, this assumption rarely matches the physical situation for proposed imaging systems. For example, several recently proposed and implemented biomedical imaging systems utilize a matching medium, or fluid, contained in a tank made of material such as plexi-glass. The assumption of a homogenous background Green's function in the inversion technique ignores the field distortions caused by the matching medium, and leads to inversion artifacts.
An improved inversion technique that matches the physical situation for proposed imaging systems by creating field distortions needed to produce an improved tomographic image. The present invention satisfies this demand.